1. Field of the Invention
The present invention relates to a lithographic printing plate precursor having good visibility of a printing plate after exposure, and to a lithographic printing method including on-press development.
2. Background Art
A lithographic printing plate generally comprises a lipophilic image area that receives ink and a hydrophilic non-image area that receives a fountain solution in printing. Lithographic printing is a printing method of making difference in ink-adhering property on the surface of a lithographic printing plate with the lipophilic image area of the lithographic printing plate as the ink-receptive area and the hydrophilic non-image area as the fountain solution-receptive area (ink-repellent area) by making use of the natures of water and oily ink of repelling to each other, adhering ink only on the image area and transferring the ink to the material to be printed, e.g., paper.
For manufacturing a lithographic printing plate, a lithographic printing plate precursor (a PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (an image-recording layer) has so far been widely used. A lithographic printing plate is generally obtained by a plate-making method of exposing a lithographic printing plate precursor through an original image of a lith film and the like, and then, for leaving the area to become an image area of the image-recording layer, dissolving and removing other unnecessary image-recording layer with an alkali developing solution or an organic solvent, to thereby bare a hydrophilic support surface to form a non-image area.
In a conventional plate-making process of a lithographic printing plate precursor, a process of dissolving and removing unnecessary image-recording layer with a developing solution and the like after exposure is necessary, but the disuse or simplification of such an additional wet process is one of the objects in the industry. Since the discard of waste solutions discharged with wet processes is a particularly great interest in the industry at large in recent years from the consideration of the global environment, the solution of the above problem is increasingly desired.
As a simple plate-making method as a countermeasure, a method that is called on-press development is proposed, which is a method of using an image-recording layer capable of removing an unnecessary area of a lithographic printing plate precursor in an ordinary printing process, and removing a non-image area after exposure on a printing press to obtain a lithographic printing plate.
As the specific examples of on-press development, e.g., a method of using a lithographic printing plate precursor having an image-recording layer soluble or dispersible with, e.g., a fountain solution, an ink solvent, or an emulsified product of a fountain solution and ink, a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket of a printing press, and a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket after weakening the cohesive strength of an image-recording layer or the adhesive strength of an image-recording layer and a support by the permeation of a fountain solution and an ink solvent are exemplified.
In the present invention, unless otherwise indicated, “development process” means a process of removing an unexposed area of an image-recording layer of a lithographic printing plate precursor by bringing into contact with a liquid (generally an alkali developing solution) to thereby bare the hydrophilic support surface with an apparatus other than a printing press (generally an automatic processor), and “on-press development” means a method and a process of removing an unexposed area of an image-recording layer of a lithographic printing plate precursor by bringing into contact with a liquid (generally printing ink and/or a fountain solution) to thereby bare the hydrophilic support surface with a printing press.
However, when a conventional image-recording layer of an image-recording system utilizing ultraviolet rays and visible rays is used, it is necessary to take methods requiring much labor, such that the exposed lithographic printing plate precursor must be stored under a completely light-shielding condition or a constant temperature condition until it is mounted on a printing press, since the image-recording layer is not fixed after exposure.
On the other hand, in recent years, digitized techniques of electronically processing, accumulating and outputting image data using a computer have prevailed, and various novel image output systems corresponding to these digitized techniques have been put to practical use. Under such circumstances, a computer-to-plate technique directly making a printing plate is attracting public attention, which is a technique of scanning exposing a lithographic printing plate precursor with high convergent radiant rays such as laser beams carrying digitized image data without using a lith film. With such a tendency, it is an important technical subject to obtain a lithographic printing plate precursor well adapted to this purpose.
As has been described, the simplification of plate making operation, and the realization of dry system and non-processing system have been more and more desired from both aspects of the global environmental protection and the adaptation for digitization.
Since high output lasers such as semiconductor lasers and YAG lasers radiating infrared rays of the wavelength of from 760 to 1,200 nm are inexpensively available nowadays, methods of using these high output lasers as image recording means are now promising as the manufacturing method of a lithographic printing plate by scanning exposure that is easy to be included in digitized techniques.
In conventional plate-making methods, image recording is carried out by imagewise exposing a photosensitive lithographic printing plate precursor by low to middle intensity of illumination to cause imagewise change of physical properties by photochemical reaction in the image-recording layer. While in the above method of using high output lasers, an exposure area is irradiated with a great quantity of light energy in an extremely short period of time, and the light energy is efficiently converted to heat energy, the heat energy causes thermal changes such as chemical changes, phase changes and morphological or structural changes in the image-recording layer, and these changes are utilized in image-recording. Accordingly, image data are inputted by light energy, e.g., laser beams, but image recording is performed in the state including the reaction by heat energy in addition to light energy. A recording system making use of heat generation by such high power density exposure is generally called heat mode recording, and the conversion of light energy to heat energy is called light/heat conversion.
Great advantages of a plate-making method using heat mode recording are that image-recording layers are insensitive to the lights of ordinary levels of illuminance such as room illumination, and that the fixation of images recorded by high illuminance exposure is not essential. That is, lithographic printing plate precursors for use in heat mode recording are free of sensitization by room illumination before exposure and fixation of images is not essential after exposure. Therefore, for example, if a lithographic printing plate precursor having an image-recording layer that is insolubilized or solubilized by exposure with high output laser beams that is capable of on-press development is available, a printing system that an image is not influenced even if exposed to room light after exposure becomes possible. That is, it is expected that a lithographic printing plate precursor preferably used for on-press development can be obtained if heat mode recording can be used.
However, many conventional photosensitive materials useful as image-recording layers in practical use have photosensitive wavelengths in the visible ray region of 760 nm or less, so that these materials cannot be used in image recording with infrared lasers. Therefore, materials capable of image recording with infrared lasers have been desired.
As one example, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image-forming layer containing hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder is disclosed in patent literature 1 (Japanese Patent 2938397). Patent literature 1 discloses that it is possible to perform on-press development with a fountain solution and/or ink by subjecting the lithographic printing plate precursor to exposure with an infrared laser to coalesce the hydrophobic thermoplastic polymer particles by heat to thereby form an image, and then mounting the lithographic printing plate precursor on the cylinder of a press.
However, there is a problem that a method of forming an image by coalescence of fine particles by mere thermal fusion as above certainly shows a good on-press developing property, but image strength (the adhesion with a support) is extremely weak and press life is insufficient.
On the other hand, lithographic printing plate precursors containing microcapsules encapsulating a polymerizable compound on a hydrophilic support is disclosed patent literature 2 (JP-A-2001-277740 (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application”)) and patent literature 3 (JP-A-2001-277742). Further, patent literature 4 (JP-A-2002-287334) discloses a lithographic printing plate precursor comprising a support having provided thereon a photosensitive layer containing an infrared absorber, a radical polymerization initiator and a polymerizable compound. A method of using a polymerization reaction is characterized in that image strength is relatively strong since the density of chemical bonding in an image area is high as compared with an image area formed by heat fusion of polymer fine particles.
In general, as the preprocess of mounting a printing plate on a printing press, the detection and discrimination of images on a printing plate, i.e., works for ascertaining whether the images fitting for the purpose are recorded on the printing plate or not, and ascertaining for what a color of ink the plate is, are operated. In ordinary lithographic printing plate precursors accompanied by a development process, an image can be easily ascertained after plate making (after development process), or before printing (before a printing plate is mounted on a printing press) generally by coloring an image-recording layer in advance.
However, in a lithographic printing plate precursor of an on-press development type or a non-processing (non-development) type not accompanied by development process before printing, the discrimination of a plate cannot be done, since there is no image on the printing plate, which sometimes leads to the error in operation. In particular in multicolor printing, it is important for printing work to be capable of distinguishing whether register marks for register are clearly written so as to be distinguished or not.
However, since an on-press development type lithographic printing plate precursor is subjected to no special process after exposure until development on a printing press, it is necessary that plate detection be done by colored or decolored images only by exposure operation. Further, for preventing the transfer of a colored matter to the printing press and a printed matter by the substance removed by on-development, in a photo-polymerization negative printing plate, a coloring system that a colorless layer is colored by exposure is preferred to a decoloring system that a colored layer is decolored by exposure, and a technique capable of not coloring a removed substance in ink and a fountain solution is desired. Further, it is desired that a colored image is not decolored and stable due to the lapse of time.
As the discoloring agent or discoloring system that causes color change by exposure, (a) compounds that themselves are discolored by any energy, e.g., heating, application of pressure or irradiation, and (b) compounds that themselves are not discolored by the application of energy but are discolored by the contact with any other component (a component that discolors a discoloring agent), are exemplified.
As the well-known examples of above (a), leuco compounds, e.g., a thermochromic compound, a piezochromic compound, a photo-chromic compound, a triarylmethane dye, a quinoline dye, an indigoid dye and an azine dye are exemplified. These compounds are discolored by the application of heat or pressure, irradiation with light or air oxidation.
As the well-known examples of the above (b), various systems (discoloration systems) that cause discoloration among two or more components, e.g., an acid-base reaction, an oxidation reduction reaction, a coupling reaction, a chelate forming reaction, are exemplified. For example, coloring systems comprising acid substances (color developers) such as acid clay and phenols with a coupler having a partial stricture of lactone, lactam, spiropyran or spirooxazine used in pressure-sensitive paper as discoloring components, systems utilizing the azo coupling reaction of aromatic diazonium salt, diazotate, diazosulfonates with naphthols, anilines, active methylenes etc., chelate-forming reactions such as the reaction of hexamethylenetetramine with ferric iron ion and gallic acid, and the reaction of phenolphthalein-Complexon acids with alkaline earth metal ions, and oxidation reduction reaction such as the reaction of ferric stearate with pyrogallol, and the reaction of silver behenate with 4-methoxy-1-naphthol are exemplified.
Further, patent literature 5 (JP-A-7-333835) discloses a photosensitive lithographic printing plate containing a photo-bleaching coloring complex comprising spiropyran and a metal salt. Patent literature 6 (JP-B-5-34392 (the term “JP-B” as used herein refers to an “examined Japanese patent publication”)) discloses a technique of coupling spiropyran having a silanol group to silica gel. However, these are techniques of systems that cause decoloration by exposure and not to obtain a colored image by exposure.
Further, patent literature 7 (JP-B-55-44935) proposes the stabilization of a spiropyran colored image by activated metal oxide. However, the patent is related to photography and copying materials using photo-chromic compounds, and on-press development type lithographic printing plate precursors using infrared lasers is not disclosed at all.
Thus, discoloration systems of the compounds that cause discoloration by exposure are known, but the systems usable in lithographic printing plate precursors capable of on-press development, excellent in a coloring property, and showing good aging stability of a colored image are not known yet.